You need to block the ridge vent before installing spray foam insulation under the roof deck.

Ask your architect.

Spray foam insulation applied to the underside of the roof sheathing is usualy done for unvented (conditioned) attics where there would be no ridge or eave vents.

The presence of a ridge vent (and eave vents) suggests there will be baffles between the rafters to create an air space between the insulation and the roof sheathing to allow ventilation.

In either case, the foam insuation would cover the ridge.

If the insulation is in the floor of the attic, the ridge and eave vents would be open to the unconditioned attic space.

Double check the design intent.

Appreciate the comments. We are changing from a vented attic design to a fully sealed conditioned attic with spray foam. Any suggestions on the best way to block the vent or will roof cap sheathing be sufficient between foam and the open air?

Hi, Ryan,

I converted the attic of my former home from a conventional (vented) attic to a conditioned attic. We left the ridge vent in place and didn't do a good job of blocking it prior to spraying the foam. Some years later wood roaches (the lot was heavily wooded) were able to enter the attic by tunneling through the foam. I removed the shingle-over ridge vent and replaced the cap shingles--which is what I should have done at the outset. That's what I suggest you do. Don't forget to block or preferably, remove any soffit vents.

^ not just roaches, but if you have water infiltration from wind (say from a vent without the foam) you will not know with closed cell until it is too late. So +1 on remove all the vents, and the roof underlayment should overlap the ridge.